Several models have been proposed to explain hydrogen-induced degradation, but only two general categories remain: hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). It is generally agreed upon that the synergism between hydrogen and metalloids at grain boundaries diminishes cohesion, leading to detrimental effects. The understanding of hydrogen-induced fracture mechanisms has evolved over time. A linear increase in Young’s modulus E with hydrogen content was reported for several metals. Lattice distortion around solute hydrogen atoms and resulted interactions between dislocations and hydrogen may cause solid solution hardening or softening. The effects of hydrogen on flow stress in materials are influenced by various factors, including hydrogen purity, grain size, specimen size, testing temperature, strain rate, and hydrogen fugacity. The decrease in flow stress and enhanced stress relaxation support the concept of hydrogen-enhanced plasticity. Blistering is induced under high hydrogen fugacity even without external stress, but degradation or eventually failure of structural steel components emerges in use under mild atmospheric environments. Crack growth rate is important for kinetics of hydrogen embrittlement. The crack growth in dry hydrogen gas proceeds with adsorption and following migration of hydrogen on the specimen surface. Not only under high-pressure hydrogen gas environments, hydrogen comes from humid or corrosive environments, during cathodic protection of off-shore structures, or at electroplating, and plays a crucial role in fatigue failure. Hydrogen-induced stress corrosion cracking is a type of damage that represents a transitional form between classical hydrogen embrittlement and stress corrosion cracking. This type of material damage should be considered as localized hydrogen embrittlement rather than a global phenomenon.